1. Field of the Invention
The present invention relates to a driving control apparatus for a vibration driven motor or actuator for performing driving control of a so-called vibration driven (vibration wave) motor comprising, e.g., electrostrictive elements, piezoelectric elements, and the like.
2. Description of the Related Art
In order to efficiently drive a vibration wave motor, it is desirable to drive the motor at a mechanical resonance frequency. In order to perform such a driving operation, conventionally, a method of controlling the frequency to obtain a constant phase difference between a signal from a monitor electrode for detecting the vibrating state of the motor, and a driving AC signal is proposed in, e.g., Japanese Patent Application No. 60-226566, (corresponding to Japanese Laid-Open Patent Application No. 62-85684).
In order to obtain various rotational speeds from the motor, the motor is generally driven at frequencies other than the resonance frequency. In this case, the motor is normally driven at a frequency higher than the resonance frequency.
More specifically, when the rotational speed of the motor is changed, a method of controlling the rotational speed of the motor by arbitrarily setting a frequency higher than the resonance frequency is adopted.
In order to detect the rotational speed of the motor, a method of calculating the rotational speed from the width of a pulse generated by a rotational speed detection member such as an encoder, which is inserted between the motor and a load, is adopted.
Upon execution of control of the motor rotational speed by the above-mentioned method, when the driving frequency is to be changed, if the rate of change in frequency is increased, then the rotational speed can be changed with quick response. However, when the frequency is close to the resonance frequency, if the frequency is changed at a large rate of change, then the changed frequency often exceeds the resonance frequency, and the motor is stopped, thus disturbing stable rotation.
When the rate of change in frequency is decreased to prevent the changed frequency from exceeding the resonance frequency, the response characteristics in a low rotational speed range of the motor are impaired, and the time required for stopping the motor is undesirably prolonged.
As a method of solving the above-mentioned problem, a conventional device disclosed in Japanese Patent Laid-Open No. 1-283073 proposes the following driving control apparatus for a vibration wave motor. In this apparatus, a monitor electrode for detecting the vibrating state of a vibration wave motor is provided and a phase difference between the output signal from the monitor electrode and a driving AC signal is detected. When the phase difference is large upon a change in rotational speed of the motor, the rate of change in frequency of the driving AC signal is increased to improve response characteristics. On the other hand, when the phase difference is small, the rate of change in frequency of the driving AC signal is decreased to prevent the motor from suddenly stopping due to a driving AC signal frequency lower than the resonance frequency.
However, since the vibration wave motor has characteristics (so-called phase return phenomenon) of exhibiting a signal value almost equal to that in a driving state at the resonance frequency or a frequency near it when the motor is driven in a high-frequency range other than the driving state at the resonance frequency or a frequency near it, the apparatus according to the conventional design erroneously determines that this driving state signal value in the high-frequency driving state is a value at the resonance frequency or a frequency near it, and decreases the rate of change in frequency, thus impairing the response characteristics obtained when the motor is decelerated or stopped.